Thermotaxis by pseudoplasmodia of Dictyostelium discoideum.

The temperature dependence of migration rate and of the thermotactic sensitivity of pseudoplasmodia of Dictyostelium discoideum has been measured. Migration rate increases with temperature to 20 degrees, is temperature insensitive from 20 degrees to 27.5 degrees, and decreases with temperature to 29 degrees, above which point migration ceases. However, pseudoplasmodia formed from cells grown at 23.5 degrees are thermotactic only from 22 degrees to 27.5 degrees. Thus, a temperature dependence of migration rate is not sufficient to explain thermotaxis. Because random lateral movements by the pseudoplasmodia have not been observed, the measurement of the temperature gradient appears to be spatial rather than temporal, with a half-maximal thermotactic response to a temperature gradient of about 0.04 degree/cm, or 0.0004 degree across an average pseudoplasmodium. Thermotactic sensitivity is adaptive, with pseudoplasmodia formed from cells grown at 20 degrees capable of thermotaxis at temperatures lower than cells grown at 23.5 degrees.